This invention relates to a residual capacity displaying device for calculating and displaying the residual capacity of a battery pack used as a power source for a VTR (video tape recorder) or the like.
A device for displaying the residual capacity of a battery pack has been disclosed, for instance, by Japanese Patent Unexamined Publication Sho-61-209371/(1986). In the conventional device, the residual capacity displaying circuit is so designed that charging or discharging current of a battery is detected and subjected to integration or subtraction, thereby to calculate and display the residual capacity of the battery. The residual capacity displaying circuit further operates as follows: When, after the battery is connected to an electrical device, and can no longer drive the electrical device, because the residual capacity of the battery is 0%, the voltage of the battery is read as a reference value. When the voltage of the battery becomes lower than the reference value thus read, the battery residual capacity value in an arithmetic control section is reset to zero (0). Hence, it is possible to correct the difference between the calculated capacity value and the actual capacity value, which may occur when the battery is repeatedly charged and discharged.
When a memory effect occurs with the battery, or when the battery is deteriorated being used for a long time, the capacity of the battery is lowered as compared with its initial capacity. Therefore, the voltage of the battery reaches the above-described reference value earlier, and the discharging time is shortened accordingly. And, similarly as in the above-described case, the battery residual capacity value in the arithmetic control section is reset. In this case, although the battery has been deteriorated, the battery residual capacity value is displayed accurately. Hence, the user may use the battery repeatedly without knowing the fact that the battery has been deteriorated. If the deteriorated battery is used for an electrical device such as for instance a VTR, then it is impossible for the battery to drive the electrical device for a required period of time, and the operation of the electrical device may be stopped unexpectedly.